116 PYKOMETER chiefly to the admixture of iridium or other metals of the same group, and it appears that the platinum prepared by the old welding pro- cess is purer and therefore better suited for electrical purposes than the metal consolida- ted by fusion in a Deville furnace. This py- rometer has already received several useful applications. Through its first application an important telegraph cable was saved from de- struction through spontaneous generation of heat. Prof. Bolzani of Kazan has made some interesting applications of it for recording the temperature at elevated points and at points below the. earth's surface. Mr. Lowthian Bell has used it in his well known researches on blast-furnace economy; and at several iron works pyrometer tubes are introduced into the heating stoves, and permanently connected with the office, where the heat of each stove can at all times be read off and recorded." Ex- perience has shown that of all pyrometers, this is the best adapted for use in the arts. 9. By Mayer's pyrometer the expansion of the wave length of a definite sound, caused by ele- vation of temperature, is measured as follows : Opposite the mouth of an organ pipe is placed a Helmholtz resonator (see SOUND), which re- sponds to the note of the organ pipe. The sonorous pulses, emanating from the organ pipe, enter the mouth of the resonator, and are thence sent through a tube terminated by a spiral tube of platinum. The pulses which have passed through this spiral tube are led to one of Konig's vibrating manometric flames. Another flame placed directly behind the for- mer one is vibrated by pulses which have pro- ceeded directly from the organ pipe. If the temperature in the organ pipe and in the spiral tube is the same (as is the case before the latter is introduced into the furnace), on viewing the flames in a rotating mirror we shall see both flames vibrating together and presenting the appearance of a deeply serrated band of light. Now, on slowly introducing the spiral tube into the furnace, we shall see the serrations, pro- duced by the pulses which have traversed this tube, slowly sliding over the fixed serrations which are caused by the pulses led directly from the organ pipe to its special flame. After the air in the spiral tube has reached the tempera- ture of the furnace and is stationary, we shall observe the serrations stationary also. From this observation of the number of movable ser- rations which have glided over any one fixed serration we can deduce the temperature of the furnace, as follows : Let t = temperature centigrade of the air in and around the organ pipe ; t' = that of the air in the spiral or fur- nace tube ; v = velocity of sound at tempera- ture t ; v' = that of sound at temperature t' ; I = number of wave lengths in furnace tube at temperature t ; d = observed displacement of resonator serrations by an elevation of tem- perature t' t. Then <', the temperature of the furnace, will be t > = PYROPHORUS which gives t' in terms of t>, Z, and d. For fuller details concerning this method see the " American Journal of Science " for Decem- ber, 1872. The advantage of this process is that no correction has to be made for baro- metric pressure, and the precision of the meth- od depends alone on the accuracy of the de- termination of the coetficient -00367, which is the number arrived at by Regnault and Mag- nus for the expansion of air under a constant pressure ; and this is one of the most certain constants we have in physics. Hence, theo- retically, this method is as accurate as that of the air thermometer. For further information on this important subject of pyromctry, see an article entitled Pyrometrische Versuche, by A. Weinhold, in Poggendorff's Annalen, vol. xxix., 1873. In this the author gives the bib- liography of the subject and details of his ex- periments with all pyrometers to decide their relative values in practice. PYROPHONE (Gr. 7rfy>, and ijxjvJj, sound), or Flame Organ, a musical instrument invented by Frederic Kastner of Paris, in which the tones are produced by flames of hydrogen or illu- minating gas burning in tubes of different sizes and lengths, arranged similarly to those in the common pneumatic organ. The pro- duction of musical tones by means of the little apparatus called the philosopher's lamp, in which hydrogen gas is burned in a tube, is a popular and familiar experiment; but it has been hitherto difficult to produce the same ef- fects with illuminating gas in consequence of the carbon element interfering with the explo- sions of the gases. Kastner has overcome this difficulty by burning the gas in several small jets arranged in a circle, instead of a large one. He also made the discovery that when these flames were brought together the sound ceased, reappearing as soon as they were sep- arated, and that the position of the flames should be one third the distance from the base of the tube. By a mechanical contrivance keys like those of a pianoforte or organ are connected with jointed arms, at the end of which the flames are burned in such a manner that they may be spread apart or joined to- gether at will by a touch of the finger. The principles involved will be treated in the arti- cle SOUND. (See also FLAME.) PYROPHORUS (Gr. rip, fire, and jtpetv, to bear), a substance which takes fire on exposure to the air. This property is possessed by sev- eral substances and mixtures specially pre- pared. Finely divided metals, as iron when reduced from the oxide at the lowest possible temperature by a current of hydrogen, exhibit it in a remarkable degree. The effect appears to be produced in all cases by rapid combina- tion of the oxidizable substance with the oxy- gen of the air. An excellent pyrophorus is produced by calcining in a close crucible 6 parts of lampblack mixed with 11 of sulphate of potash ; the product is a mixture of carbon and sulphuret of potassium. Homberg's pyro-
